An optical transmission system comprises an optical transmitter converting an electrical signal to be transmitted into optical form, (b) an optical fibre serving as a conductor for the optical signal, and (c) an optical receiver detecting the transmitted optical signal and converting it into electrical form.
A typical optical receiver comprises at its input stage a photo detector and a transimpedance amplifier whose input is connected to the photo detector. The photo detector converts the optical signal it has received into electric current that is supplied to the transimpedance amplifier. The latter generates at its output a voltage proportional to the incoming current, and thus a voltage proportional to the current of the photo detector is obtained from the output of the amplifier. The photo detector is usually either an avalanche photo diode (APD) or an optical PIN diode. Avalanche photo diodes are typically used at shorter wavelengths and optical PIN diodes at longer wavelengths, at which avalanche photo diodes generate a considerable amount of noise. Transimpedance amplifiers are generally used for example for the reason that they allow comparatively good sensitivity properties to be achieved with a relatively simple construction.
Notwithstanding transimpedance amplifiers, one problem in optical receiver solutions lies in their poor dynamics: good sensitivity often entails a poor power tolerance and a good power tolerance again poor sensitivity. Poor dynamics for their part impair the operational flexibility of the receiver; for example when beginning to use a shorter fibre, an extra attenuator must be added between the transmitter and the receiver.
Since the power level of the optical signal arriving at the receiver can in practice vary a great deal (depending on how long fibre is used), automatic gain control (AGC) is typically used in connection with the transimpedance amplifier to keep the amplifier's output voltage essentially at a constant value, when the incoming signal is higher than a predetermined threshold value.
When good sensitivity is aimed at, the stray capacitances on the input terminal of the amplifier are significant; even a small capacitance will impair the sensitivity of the receiver. Hence, it is essential that the parasitics on the input of the amplifier can be minimized.
An attempt has been made to widen the dynamic range of the receiver by using an adjustable resistive element in front of the transimpedance amplifier. The resistance of the element is adjusted in response to the strength of the signal arriving at the amplifier in such a way that at higher levels the resistance is diminished, as a result of which the current coupled to the input of the amplifier will diminish (part of the current passes through the resistive element) and the amplifier is not saturated. This basic solution is known in several different variations, which will be briefly described in the following.
A control circuit is presented in U.S. Pat. No. 5,012,202 and in EP Patent Publication 433 646-B1, wherein a field-effect transistor (FET) is used as the resistive element. To prevent the drain capacitance of the field-effect transistor from reducing the amplifier's sensitivity, it must be compensated for with a feedback over the field-effect transistor. Such a feedback, however, makes the circuit even more complicated. In addition, the feedback makes it more difficult to design the receiver.
An alternative that is better than the field-effect transistor is to use a diode with a naturally low capacitance as the adjustable resistive element. There are several different solutions based on a diode.
GB Patent Application 2 247 798-A presents a diode-based solution, wherein based on a voltage formed over a resistor (r, FIG. 1) a switching transistor (TR1, FIG. 1) is used to control a voltage over a diode (D, FIG. 1) and thus to control the dynamic resistance of the diode. To prevent the control circuit from interfering with the DC operating point of the transimpedance amplifier, a capacitor (C2, FIG. 1) must be used to separate it from the amplifier. However, the use of a capacitor causes an additional time constant in the feedback loop, which complicates the design. In addition, the capacitors and the above-mentioned resistor make the circuit more complex, whereby more space than before is also required on the circuit board. All additional components also cause parasitics at high frequencies, which reduces the sensitivity of the receiver.
U.S. Pat. No. 4,415,803 also describes a diode-based solution using such a peak-hold circuit in the control which monitors the peak value of the transimpedance amplifier output. Based on this peak value, the voltage over the diode is controlled so that a part of the amplifier's input current will pass through the diode. A drawback of this solution is that it is difficult to bring about an exact peak-hold circuit at high transmission rates. It is difficult to achieve a great precision in peak value measurements and, besides, expensive special components must be used.
A solution is known from EP Patent Application 402 044-B1, wherein the detector current passes through a resistor (R1, FIGS. 4a and 4b) and causes a voltage over the diode (D0, FIGS. 4a and 4b). When this voltage is sufficiently high, the diode begins to conduct, whereby it functions as an attenuator. One drawback of this solution is that the resistor causes noise, which again reduces the sensitivity of the receiver.